1. Field of the Invention
This invention relates generally to the dispersing of liquids into fluidized solids. More specifically this invention relates to a method and apparatus for atomizing liquid into fine droplets and dispersing the droplets into a suspension of fluidized solids. A specific aspect of this invention relates to the contacting of fluidized catalyst particles with a liquid hydrocarbon wherein the liquid hydrocarbon is atomized into a dispersion of fine droplets and injected into the stream of fluidized catalyst particles.
2. Description of the Prior Art
There are a number of continuous cyclical processes employing fluidized solid techniques in which carbonaceous materials are deposited on the solids in the reaction zone and the solids are conveyed during the course of the cycle to another zone where carbon deposits are at least partially removed by combustion in an oxygen-containing medium. The solids from the latter zone are subsequently withdrawn and reintroduced in whole or in part to the reaction zone.
One of the more important processes of this nature is the fluid catalytic cracking (FCC) process for the conversion of relatively high-boiling hydrocarbons to lighter hydrocarbons boiling in the heating oil or gasoline (or lighter) range. The hydrocarbon feed is contacted in one or more reaction zones with the particulate cracking catalyst maintained in a fluidized state under conditions suitable for the conversion of hydrocarbons.
It has been a long recognized objective in the FCC process to maximize the dispersal of the hydrocarbon feed into the particulate catalyst suspension. Dividing the feed into small droplets improves dispersion of the feed by increasing the interaction between the liquid and solid. Preferably, in hydrocarbon conversion droplet sizes become small enough to permit vaporization of the liquid before it contacts the solids.
It is well known that agitation or shearing can atomize a liquid hydrocarbon feed into fine droplets which are then directed at the fluidized solid particles. A variety of methods are known for shearing such liquid streams into fine droplets.
U.S. Pat. No. 3,071,540 discloses a feed injection apparatus for a fluid catalytic cracking unit wherein a high velocity stream of gas, in this case steam, converges around the stream of oil upstream of an orifice through which the mixture of steam and oil is discharged. Initial impact of the steam with the oil stream and subsequent discharge through the orifice atomizes the liquid oil into a dispersion of fine droplets which contact a stream of coaxially flowing catalyst particles.
U.S. Pat. No. 4,434,049 shows a device for injecting a fine dispersion of oil droplets into a fluidized catalyst stream wherein the oil is first discharged through an orifice onto an impact surface located within a mixing tube. The mixing tube delivers a cross flow of steam which simultaneously contacts the liquid. The combined flow of oil and steam exits the conduit through an orifice which atomizes the feed into a dispersion of fine droplets and directs the dispersion into a stream of flowing catalyst particles.
The injection devices of the '540 and '049 patents rely on relatively high fluid velocities and pressure drops to achieve atomization of the oil into fine droplets. Providing this higher pressure drop burdens the design and increases the cost of equipment such as pumps and exchangers that are typically used to supply liquid and gas to the feed injection device. The need to replace such equipment may greatly increase the cost of retrofitting an existing liquid-solid contacting installation with such an injection apparatus.
Other methods for atomizing liquid feeds with gaseous material are shown in U.S. Pat. Nos. 3,152,065 and 3,654,140. FIG. 2 of U.S. Pat. No. 3,654,140 shows an injection device that imparts a tangential velocity to an oil stream to promote its mixing with a stream of steam which is injected into the oil outside the injection device. In U.S. Pat. No. 3,152,065 an injection device adds a tangential velocity to an annular stream of oil that flows around a central conduit. Steam passing through the center conduit contacts the oil at the distal end of the injector. Steam and oil then pass through an orifice which further atomizes the oil and distributes it into a dispersion of fine droplets. In these devices, the tangential velocity of oil in combination with the expansion of the steam is relied on to provide the energy for atomizing the oil.
U.S. Pat. No. 4,717,467 shows a method for injecting an FCC feed into an FCC riser from a plurality of discharge points. The discharge points in the '467 patent do not radially discharge the feed mixture into the riser.
Another useful feature for dispersing feed in FCC units is the use of a lift gas to pre-accelerate the catalyst particles before contact with the feed. Modern FCC units use a pipe reactor in the form of a large, usually vertical, riser in which a gaseous medium upwardly transports the catalyst in a fluidized state. Catalyst particles first enter the riser with zero velocity in the ultimate direction of riser flow. Initiating or changing the direction of particle flow creates turbulent conditions at the bottom of the riser. When feed is introduced into the bottom of the riser the turbulence can cause maldistribution and variations in the contact time between the catalyst and the feed. In order to obtain a more uniform dispersion, the catalyst particles are first contacted with a lift gas to initiate upward movement of the catalyst. The lift gas creates a catalyst pre-acceleration zone that moves the catalyst along the riser before it contacts the feed. After the catalyst is moving up the riser it is contacted with the feed by injecting the feed into a downstream section of the riser. Injecting the feed into a flowing stream of catalyst avoids the turbulence and backmixing of particles and feed that occurs when the feed contacts the catalyst in the bottom of the riser. A good example of the use of lift gas in an FCC riser can be found in U.S. Pat. No. 4,479,870 issued to Hammershaimb and Lomas.
The addition of lift gas to initially accelerate the catalyst can also be used for a variety of purposes such as treating the catalyst particles prior to contact with the feed and varying the residence time of the feed in the riser. There are many references which teach, for various reasons, the mixing of hot regenerated FCC catalyst with various relatively light materials prior to contact of the catalyst with the FCC feedstock. Thus, in U.S. Pat. No. 3,042,196 to Payton et al. beginning with a light cycle oil, progressively heavier components are added to an upflowing catalyst stream in a reactor riser so as to use a single catalyst and a single cracking zone to convert the elements of a crude oil. In U.S. Pat. No. 3,617,497 to Bryson et al. a light gas oil is mixed with a diluent vapor such as methane or ethylene at or near the bottom of a reactor riser with hot regenerated catalyst, introduced at the same point in the riser or very close downstream, with the mixture then contacted with heavy gas oil at the top of the riser so as to enhance gasoline yield. In U.S. Pat. No. 3,706,654 to Bryson et al., naphtha diluent may be added to the bottom of a reactor riser to aid in carrying upwardly into the riser the regenerated catalyst stream. In U.S. Pat. No. 3,849,291 to Owen it is disclosed that a gasiform diluent material comprising C.sub.4 + hydrocarbons and particularly C.sub.5 + hydrocarbons may be used to form a suspension with freshly regenerated catalyst which suspension is caused to flow through an initial portion of a riser reactor before bringing the hydrocarbon reactant material in contact therewith in a downstream portion of the reactor so as to achieve a very short residence time (1 to 4 seconds) that the hydrocarbon is in contact with the catalyst suspension in the riser reactor (catalyst residence time). U.S. Pat. No. 3,894,932 to Owen discusses contacting the FCC conversion catalyst with a C.sub.3 -/C.sub.4 rich hydrocarbon mixture or an isobutylene rich stream before contact with gas oil boiling range feed material in an initial portion of the riser (catalyst to hydrocarbon weight ratio from 20 to 80) so as to upgrade the C.sub.3 -C.sub.4 material to a higher boiling material. U.S. Pat. No. 4,422,925 to Williams et al. discusses passing a mixture of hydrocarbons, such as ethane, propane, butane, etc., and catalyst up through a riser reactor at an average superficial gas velocity within the range from about 40 to about 60 feet per second (12.2-18.3 meters/sec), with a catalyst to hydrocarbon weight ratio of about 5 to about 10 so as to produce normally gaseous olefins. In U.S. Pat. No. 4,427,537 to Dean et al. there is shown catalyst particles mixed with a fluidizing gas, such as a gaseous hydrocarbon, charged to a bottom portion of a reactor riser to promote or provide for a smooth non-turbulent flow up the riser of a relatively low velocity dense flow of catalyst particles.
There are additional references which show use of a lift gas in non-catalytic systems. For example, in U.S. Pat. No. 4,427,538 to Bartholic, a gas which may be a light hydrocarbon is mixed with an inert solid at the bottom part of a vertical confined conduit and a heavy petroleum fraction is introduced at a point downstream so as to vary the residence time of the petroleum fraction in the conduit. Similarly, in U.S. Pat. No. 4,427,539 to Busch et al., a C.sub.4 minus gas is used to accompany particles of little activity up a riser upstream of charged residual oil so as to aid in dispersing the oil.
The use of feed atomization and lift gas, to pre-accelerate catalysts, have been combined in FCC risers to obtain the benefit of a more uniformly dispersed feed across the cross section of a riser reaction zone. While both the catalyst pre-acceleration obtained from lift gas and feed atomization are desirable features for improving feed injection, these features are usually combined in a manner that requires a relatively long contact time before the feed is completely mixed with the catalyst particles. The typical devices for the atomization of the feed will inject the feed into the riser at high velocities typically greater than 30 meters per second. It has been the practice to aim the exit stream from the atomization device in a principally parallel direction to the flow of catalyst particles. Since the catalyst and gas mixture passing by the distribution nozzles has a typical velocity of less than 12 meters per second the feed and catalyst must travel along the riser for a distance called the axial contact length before the feed and catalyst achieve a uniform velocity and thorough mixing. The atomization nozzles are usually positioned with their nozzles near the wall of the riser. Positioning the feed nozzles near the wall of the riser keeps pipe elements out of the central portion of the riser which would introduce turbulence and undo part of the work of the catalyst pre-acceleration zone. Part of the non-uniformity in the mixing over the axial contact zone is the result of feed having to travel from the wall of the riser to the center of the riser. As the length of the axial contact zone is decreased, better initial mixing of the catalyst and feed results. Therefore, it would be highly desirable to have a method and apparatus for contacting FCC catalysts with a hydrocarbon feed that includes the features of catalyst pre-acceleration feed atomization and minimum axial contact zone length.